Varnish: fix wikimedia.vcl.erb's std.collect()

Breakage introduced with I0bf908; import std to use std.collect(). Also
make this run only on frontends for efficiency.

Change-Id: I868bf1a8c12b64e1f76565d302cc0ba9f7a6df0d
Merge "Varnish: fix wikimedia.vcl.erb's std.collect()" into production